ICs as semiconductor devices, which term also includes transistors, LSIs and VLSIs, are typically fabricated by the following steps:
(a) provide a lead frame material that is made of a strip of Cu alloy having a thickness of 0.1-0.3 mm; PA0 (b) etch or stamp out a lead frame conforming to the shape of the ICs to be fabricated; PA0 (c) apply high-purity Si or Ge semiconductor chips to selected areas in the lead frame by thermocompression with an electrically conductive resin such as Ag paste, or by soldering with a plating of Au, Ag, Ni, Cu or an alloy thereof formed on the mating surfaces of each semiconductor chip and the lead frame, or by Au brazing; PA0 (d) perform ball bonding to connect each semiconductor chip to the lead frame by bridging them with very fine (20-50 .mu.m diameter) Au wires; PA0 (e) enclose with a protective plastic package the semiconductor chips, bonded gold wires and parts of the lead frame to which the chips have been bonded; and PA0 (f) cut the lead frame into discrete ICs.
Since the Au wires conventionally employed as bonding wires in the fabrication of semiconductor devices are expensive, substantial reduction in the manufacturing cost can be realized by replacing the Au wires with much less expensive Cu wires. In order to materialize this possibility, attempts are being made to use bonding wires made of oxygen-free copper and other high-purity copper materials having comparatively low hardness values.
By repeated cycles of electrorefining, the purity of copper materials can be increased to at least 99.999% (5-nines or 5 N purity). However, even if this high-purity copper material after being further purified by 20-30 cycles of zone melting, is annealed to the fullest extent, its Vickers hardness (Hv) lies in the rang of about 45 to 48 and is not as low as the value for the softer Au (Hv=30). If high-speed ball bonding is performed with this relatively hard Cu wire (0.15-0.3 second intervals between bonding cycles are common with very fine Au wires), the ball forming at the tip of the wire may break the bonding pad of Al alloy on the semiconductor chip, or may even sometimes cause microcracking in the chip.